Read/write head for optical disk drive and optical disk drive comprising such a read/write head

ABSTRACT

A read/write head for a optical disk drive, comprises a lens holder ( 8 ), a support frame ( 11 12, 16 ), means ( 15 ) for suspending the lens holder ( 8 ) in the support frame which means constrain movement of the lens holder ( 8 ) relative to the support frame, allowing only an at least limited translation in a focussing direction (z), parallel to the optical axis of a lens ( 7 ) in the lens holder ( 8 ), an at least limited translation in a tracking direction (y), perpendicular to the focussing direction (z), and an at least limited rotation about an axis in a tangent direction (x), perpendicular to both the focussing and the tracking direction, and actuator means, comprising only two conductive focussing coils ( 9, 10 ) with a winding axis parallel to the focussing direction (z), each positioned relative to a magnetic circuit ( 11, 12 ) in such a way that a current flowing through a coil ( 9, 10 ) gives rise to a force between the lens holder ( 8 ) and the support frame in the focussing direction (z). The winding axes of the two focussing coils ( 9, 10 ) are positioned on opposite sides of a plane through the center of mass of the lens holder ( 8 ), and parallel to the focussing and the tangent direction. The focussing coils ( 9, 10 ) are spaced apart in the tangent direction (x).

The invention relates to a read and/or write head for an optical diskdrive, comprising a lens holder, a support frame, means for suspendingthe lens holder in the support frame, which means constrain movement ofthe lens holder relative to the support frame, allowing only an at leastlimited translation in a focussing direction, parallel to the opticalaxis of a lens in the lens holder, an at least limited translation in atracking direction, perpendicular to the focussing direction, and an atleast limited rotation about an axis in a tangent direction,perpendicular to both the focussing and the tracking direction, andactuator means, comprising two conductive focussing coils with a windingaxis parallel to the focussing direction, each positioned relative to amagnetic circuit in such a way that a current flowing through a coilgives rise to a force between the lens holder and the support frame inthe focussing direction, the winding axes of the two focussing coilsbeing positioned on opposite sides of a plane through the center of massof the lens holder and parallel to the focussing and the tangentdirection.

The invention also relates to an optical disk drive comprising such aread/write head.

An example of a read/write head of the type mentioned in the openingparagraph is known from U.S. Pat. No. 5,905,255. This document disclosesan embodiment of an objective lens driver comprising a movable member.An objective lens, a lens holder, and a first and a second permanentmagnet, fixed on the lens holder, constitute the movable member. Theobjective lens and the first and the second permanent magnet aredisposed symmetrically about a plane of symmetry which extends throughthe center of gravity of the lens holder and is parallel to thefocussing direction and the tangent direction. Four wire members arestretched parallel to the tangent direction between a holding member andthe lens holder. First and second bobbins are vertically disposed on afixed base so that they are located side by side in the trackingdirection. They are formed by yokes comprising a magnetic plate,extending in the focussing direction and the tracking direction, aroundwhich are wound tracking coils having the winding axis in the trackingdirection and focussing coils having the winding axis in the focussingdirection.

The known arrangement has the disadvantage that the dimensions of themagnetic plates, the winding of the focussing coil around the trackingcoil, and the positioning of the bobbins side by side in the trackingdirection lead to a relatively large dimension in the trackingdirection. In an optical disk drive, the read/write head is moved in aradial direction relative to the disk to be read or written. Itsorientation is such that the tracking direction is aligned in thisradial direction. As large an area of the disk as possible should beused for recording and reading data. However, the lens of a read/writehead with large dimensions in the tracking direction cannot be movedclose to the axis of rotation of the disk, since the driving arrangementused to rotate the disk will interfere with such a lens holder.

It is an object of the invention to provide a read and/or write head ofthe kind described in the preamble and a disk drive provided with such ahead, wherein the position and inclination of the lens holder can becontrolled, and which can have reduced dimensions in the trackingdirection.

This object is achieved by the read/write head according to theinvention, which is characterized in that the focussing coils are spacedapart in the tangent direction.

Because the focussing coils are spaced apart in the tangent direction, amore compact arrangement can be obtained, since the distance between thewinding axes of the coils in the tracking direction can be made smallerthan the outer dimensions of the coils without the coils making contact.At the same time, due to the arrangement of the coils on opposite sidesof the plane parallel to the focussing and the tangent direction, atilting action can be obtained when the two focussing coils are drivenin anti-phase. The focussing coil configuration will generally berestricted to said two focussing coils.

An arrangement wherein the focussing coils are spaced apart in thetangent direction is known. However, in this known arrangement thefocussing coils are both centered on the plane parallel to the focussingand the tangent direction, so that no tilting action is possible.

Preferably, the two focussing coils are point-symmetrically arrangedrelative to the center of mass of the lens holder.

This step reduces the susceptibility to excitation of resonancefrequencies during operation of the read/write head.

Preferably, the two focussing coils are mounted on the lens holder.

This has the advantage that the influence of external electromagneticfields, such as those of the motor driving the disk to be read fromand/or written to, on the position and orientation of the lens holder isvery small.

Preferably, each magnetic circuit comprises a yoke extending at leastpartly through the corresponding focussing coil along its winding axis.

This is a very compact arrangement, which takes up little space.

Preferably, each magnetic circuit forms a loop in a plane parallel tothe focussing and the tangential direction and comprises an air gapthrough which the windings of the corresponding focussing coil can move,at least one radial coil being mounted on the lens holder and located ineach air gap with a winding axis aligned with the flux through themagnetic circuit.

Thus, translations in the tracking direction can be controlled. Only onemagnetic circuit is used on each side of the lens holder for all threetypes of actuation mentioned. The manner in which focussing coils andradial coils are driven determines the position and orientation of thelens holder, that is, within the constraints provided by the means viawhich the latter is suspended in the support frame.

According to a further aspect of the invention, there is provided anoptical disk drive comprising a read/write head according to any one ofthe claims 1-10.

In such a disk drive, disks can be read and/or written close to thecenter of rotation of the disk due to the compact dimensions of theread/write head in the tracking direction.

The invention will now be explained in further detail with reference tothe accompanying figures, in which:

FIG. 1, is a diagram of the read/write head during reading from and/orwriting to a disk;

FIG. 2, is a diagram of the read/write head; and

FIG. 3, is a diagram of a circuit enabling control of the orientationand position of the lens holder.

In an optical disk drive, information is encoded in one or more layersof an optical disk 1, which is shown schematically in FIG. 1. Variousprinciples are known, each variant being suitable for use in conjunctionwith the invention. The information is laid down in one or moreinformation tracks 2 in digital form. The variation of (optical)properties along the information track 2 contains the informationrecorded on the disk 1. To read the disk 1, it is rotated by means of adisk drive motor 3. The disk 1 is read by detection of light reflectedin the disk 1.

In the example of the FIGS. 1 and 2, a light beam 4 is reflected in thedirection of the disk 1 by means of a mirror 5 which is part of aread/write head 6. This description will use the term read/write head,since many disk drive systems allow information to be written to thedisk 1 optically. The light beam 4 will then have a different powerlevel and/or wavelength, but must also be focussed onto a point in thedisk l, as is the case when the disk 1 is being read. The read/writehead 6 is intended for focussing a beam onto the disk 1 for eitherreading or writing or both. Parts of the read/write head 6 that arerelevant to the explanation of the invention are shown in the FIGS. 1and 2. Light reflected by the mirror 5 is focussed onto the disk 1 bymeans of an objective lens 7 situated in a lens holder 8.

In a typical optical disk system, the information tracks 2 are veryclosely spaced in the radial direction, in order to fit as muchinformation as possible onto the disk 1. In a Compact Disk (CD) thedistance is 1.6 μm, while in a Digital Versatile Disk (DVD) the distanceis 0.74 μm. There is a tendency towards smaller track distances in newersystems, as sources of (laser) light of smaller wavelengths andobjective lenses 7, or lens systems, with a higher numerical aperturebecome available. In the configuration shown in FIG. 1, the light beam 4is aligned in a radial direction relative to the disk 1. The positionand orientation of the mirror 5 and the objective lens 7 determine thepoint on the disk 1 at which the light is focussed. Smaller distancesbetween successive information tracks 2 are made possible by moreaccurate actuator arrangements for controlling the position andorientation of the read/write head 6.

The position of the read/write head 6 as a whole in the radial directionof the disk 1 may be controlled by means of a worm wheel acting on aslide or sledge (not shown) and driven by a sledge motor (not shown).However, fine-tuning of the position of the focussing point in the disk1 is then carried out by adjusting the position of the lens holder 8relative to the rest of the read/write head 6. To this end, theread/write head 6 comprises a support frame which is fixed to or formspart of the sledge.

The lens holder 8 is suspended in the support frame in such a way thatits movement relative to the support frame is constrained. Referring toFIG. 1, the lens holder 8 is first of all capable of carrying outtranslations in a focussing direction z. That is, it can be moved closeror further away from the disk 1. In this way, the exact point in thedisk 1 on which the light is focussed can be adjusted. Secondly, thelens holder 8 can carry out translations in a tracking direction y. Byvarying the position of the lens holder 8 in the tracking direction, theposition on which the light beam 4 is focussed can be moved further orcloser to the center of the disk 1. Thirdly, the lens holder 8 can betilted, i.e. it can carry out rotations about a tangent direction x. Inthis way, the light beam 4 can be focussed on the disk 1 in such a waythat it is always locally perpendicular to the surface of the disk,despite any inclination of the disk.

The adjustment of the position and the orientation of the lens holder 8is used to adjust for small geometric deviations in the disk 1 or in theinformation track 2. In particular, deviations from a perfect plane—an“umbrella-like” shape—can be compensated for by varying the degree oftilt and the position in the focussing direction. The possibility oftranslating the lens holder 8 in the tracking direction y makes itpossible to compensate for deviations from a spiral or circular shape ofthe information track 2. This becomes more important as a lens 7 with ahigher numerical aperture is used. Such a lens can be positioned closerto the disk and makes it possible to read a disk 1 with narrow andclosely spaced information tracks 2.

For accurate control of the position and the orientation, the read/writehead 6 comprises an actuator arrangement and a control circuit (notshown). The control circuit provides the driving signals for theactuator arrangement. It is not considered part of the invention and amultitude of possible implementations of a control circuit for thispurpose are known, so that no further description is given of thecontrol circuit.

The actuator arrangement comprises only two focussing coils, viz. afirst focussing coil 9 and a second focussing coil 10. The winding axisof each coil is parallel to the focussing direction z. The focussingcoils 9, 10 are fixed to the lens holder 8. A magnetic circuit isprovided for each of the focussing coils 9, 10. This magnetic circuitcomprises a yoke 11 and a permanent magnet 12. Of course, a yoke and anelectromagnet could also be used in principle.

A current flowing through one of the focussing coils 9, 10 will giverise to a force in the focussing direction z. Turning to FIG. 2, it willbe more clearly appreciated that the first and second focussing coils 9,10 are positioned on opposite sides of a plane through the center ofmass of the lens holder 8 and parallel to the focussing direction z andthe tangent direction x. The dashed line I lies in this plane. Due tosuch positioning, an unbalance between the forces generated when thefirst and second focussing coils 9, 10 are driven will result in atilting action of the lens holder 8.

It will be apparent from FIG. 2 that the first and second focussingcoils 9, 10 are quite large relative to the dimensions of the lensholder 8. The windings must be quite large to capture enough magneticflux to generate the required force. Alternatively, of course, theheight of the coils 9, 10 and the number of windings could be increased,but this is undesirable since the lens holder 8 must be as flat aspossible and should have a low mass. The current could also beincreased, but this would lead to the development of heat and,consequently, a lower efficiency. To achieve a compact lens holder 8,therefore, the first and second focussing coils 9, 10 are spaced apartin the tangent direction x, that is in this case at opposite ends of thelens holder 8. This enables the winding axes to be positioned closer tothe plane through the center of mass of the lens holder 8. Note that thedistance d between the plane through the center of mass of the lensholder 8 and the line 1 is smaller than the distance from the windingaxis to the outer edge of the winding of the focussing coils 9,10 in alateral direction parallel to the tangent direction. This is onlypossible because the focussing coils 9, 10 are spaced apart in thetangent direction. The compact construction of the read/write head 6makes it possible to move the read/write head 6 closer to the center ofthe disk 1. This means that a larger area of the disk l close to thecenter becomes available for storing information.

As may be summarized from FIG. 2, the focussing coils 9, 10 arepoint-symmetrically arranged relative to the center of mass of the lensholder 8. They are not only arranged at equal distances to a planethrough the center of mass and parallel to the focussing direction z andthe tangent direction x, but also at equal distances to a plane throughthe center of mass and parallel to the focussing direction z and thetracking direction y. Although measures, to be described below, havebeen taken to dampen oscillation of the lens holder 8, the lens holder 8and the suspension thereof can be viewed as a spring-mass system withcertain resonance frequencies. The generation of parasitic oscillationsis more effectively suppressed by arranging the actuators such thatforce is symmetrically applied to the lens holder 8.

An arrangement other than that of FIGS. 1 and 2, wherein the focussingcoils 9, 10 are mounted on the support frame and the permanent magnets12, with or without a yoke, are mounted on the lens holder 8, is alsopossible. This may be necessary if large currents are needed and heatcannot be effectively transferred from the lens holder 8. However, fromthe point of actuator control it is desirable to use the arrangementshown in the drawings. The permanent magnet 12 is susceptible to forcesgenerated by stray electromagnetic fields, such as those generated bythe disk drive motor 3. If permanent magnets were to be mounted on thelens holder 8, the lens holder 8 would move in an uncontrollable wayunder the influences of such stray fields. In addition, the shownarrangement is generally lower in weight, making the lens holder 8 moreresponsive, which is desirable from the point of view of controlling itsposition and orientation. Furthermore, it has become apparent inpractice, that a configuration in which the lens holder 8 carries thecoils 9, 10 is more efficient if the heat dissipation can be controlled.

The arrangement of each of the two magnetic circuits formed by the yokes1 and permanent magnets 12 is very compact and efficient. In thearrangement of the invention, almost all of the flux generated by thepermanent magnet 12 is concentrated in the yoke 11 which is made of amaterial with a high magnetic permeability. The yokes 11 extend throughthe corresponding focussing coil along its winding axis. As is apparentfrom the FIGS. 1 and 2, the magnetic circuit comprises an air gap 13.The air gap 13 defines the face of the yoke where the flux leaves theyoke. The windings of the first and second focussing coils 9, 10 thusintersect the flux passing from the face of the yoke through the air gapto the magnet 12. Because the flux is conducted through the center ofthe coils 9, 10, maximum interaction with the current flowing throughthe coils 9, 10 is ensured.

According to the invention, the same magnetic circuit is also used forthe actuator arrangement used to control movement in the trackingdirection y. The magnetic circuit forms a loop in a plane parallel tothe focussing direction z and the tangential direction x. The flux is,therefore, also parallel to the tangential direction x at a point in thecircuit.

The air gap 13 also provides space for accommodating two radial coils 14which are mounted side by side in the tracking direction y in the airgap 13 in each magnetic circuit, their winding axis being aligned in thetangential direction x. The radial coils 14 and the magnetic circuitform an actuating arrangement for controlling the position of the lensholder 8 in the tracking direction y. Although it would be possible touse only one radial coil 14, it is preferred to use two, because themagnetic flux through a single radial coil 14 would vary as the lensholder 8 changes position in the tracking direction y. A more elaboratecontrol circuit would be needed to take account of this variation of themagnetic flux density. In the current configuration, as one of theradial coils 14 moves out from between the end faces of the yoke 11 andthe permanent magnet 12, the other moves more fully into the air gap 13for compensation. The total flux through the two radial coils 14 remainssubstantially the same.

The preferred means whereby the lens holder 8 is suspended in thesupport frame of the read/write head 6 are formed by four wire members15. Each is fixed to the lens holder 8 at one end, and to a supportframe part 16 at the other end. The wire members 15 are made of aresilient material, which is preferably electrically conductive, e.g.copper, iron, or an alloy.

The wire members 15 limit the number of degrees of freedom of the lensholder 8. Only translations in the tracking direction y and thefocussing direction z are possible. Only tilt about the tangentdirection x is allowed. In particular, tilt about the focussingdirection z and the tracking direction y is not possible.

The wire members 15 are preferably electrically conductive so that theycan be used for applying driving currents to the radial coils 14 and thefirst and second focussing coils 9, 10. Turning to FIG. 3, it will beseen that four wire members 15 exactly suffice for providing therequired driving currents. The control circuit (not shown) providesthree control signals to the actuator arrangement. A radial coil controlsignal 17 determines movement in the tracking direction; the directionof the driving current then determines whether this movement is towardsor away from the center of the disk 1. A focus control signal 18controls the focussing of the beam 4 by the objective lens 7 through theposition of the lens holder 8 in the focussing direction z. A tiltcontrol signal 19 controls the degree and direction of tilt of the lensholder 8. The tilt control signal 19 is added to the focus controlsignal 18 for the first focussing coil 9, and subtracted for the second10, to obtain the driving current. Thus, the first and second focussingcoils 9, 10 are provided with different driving currents to enable tilt.The radial coils 14 are all provided with the same driving current. Theyare, therefore, connected in series. One of the conductive wire members15 is a common wire whereto the series-connected radial coils 14 andeach of the focussing coils 9, 10 are connected at one end. Current tothe radial coils 14 is supplied through a second wire member 15. Currentto the first and second focussing coils 9, 10 is supplied through athird and a fourth wire member 15.

Preferably, the wire members 15 are provided with a cladding of anelastic, preferably electrically insulating material. Apart from theinsulation, the function of the cladding is to dampen any parasiticoscillations of the lens holder 8 which, as mentioned, forms aspring-mass system with the wire member 15. Thus, more accuratepositioning is achieved.

The invention is not limited to the above embodiments which may bevaried within the scope of the claims. For example, it is not strictlynecessary that the lens holder be suspended by rod-shaped wire members.Blades shaped to form hinges could also be used, but would be muchstiffer, thus necessitating a larger force to tilt the lens holder.Further, although a single objective lens 7 is used in the describedembodiment, the lens holder may comprise a more elaborate optical systemfor focussing and/or splitting the beam, depending on the complexity ofthe optical drive.

1. A read and/or write head for an optical disk drive, comprising a lensholder, a support frame, means for suspending the lens holder in thesupport frame, which means constrain movement of the lens holderrelative to the support frame, allowing only an at least limitedtranslation in a focussing direction (z), parallel to the optical axisof a lens in the lens holder, an at least limited translation in atracking direction (y), perpendicular to the focussing direction (z),and an at least limited rotation about an axis in a tangent direction(x), perpendicular to both the focussing and the tracking direction, andactuator means, comprising two conductive focussing coils with a windingaxis parallel to the focussing direction (z), each positioned relativeto a magnetic circuit in such a way that a current flowing through acoil gives rise to a force between the lens holder and the support framein the focussing direction (z), the winding axes of the two focussingcoils being positioned on opposite sides of a plane through the centerof mass of the lens holder and parallel to the focussing and tangentdirection, characterized in that the focussing coils are spaced apart inthe tangent direction (x).
 2. A read and/or write head according toclaim 1, wherein the distance (d) between each winding axis of afocussing coil and the plane through the center of mass of the lensholder, and parallel to the focussing and the tangent direction, issmaller than the distance from the winding axis to the winding of eachfocussing coil in a lateral direction parallel to the tangent direction.3. A read and/or write head according to claim 1, wherein the twofocussing coils are point-symmetrically arranged relative to the centerof mass of the lens holder.
 4. A read and/or write head according to anyone of claims 1, wherein the two focussing coils are mounted on the lensholder.
 5. A read and/or write head according to claim 1, wherein eachmagnetic circuit comprises a yoke extending at least partly through thecorresponding focussing coil along its winding axis.
 6. A read and/orwrite head according to claim 5, wherein each magnetic circuit forms aloop in a plane parallel to the focussing and the tangential directionand comprises an air gap through which the windings of the correspondingfocussing coil can move, at least one radial coil being mounted on thelens holder and located in each air gap with a winding axis aligned withthe flux through the magnetic circuit.
 7. A read and/or write headaccording to claim 6, wherein two radial coils are mounted side by sidein the tracking direction (y) in the air gap in each magnetic circuit.8. A read and/or write head according to claim 1, wherein the suspensionmeans comprise four wire members, each attached at one end to the lensholder and at the other end to the support frame.
 9. A read and/or-writehead according to claim 4, wherein the wire members are of anelectrically conducting material and are electrically connected to thecoils.
 10. A read/write head according to claim 8, wherein the wiremembers are provided with a cladding of an elastic material.
 11. Anoptical disk drive comprising the read/write head according to claim 1.